It has been known to crop or shear the nap of textile fabrics by knives of helical form disposed on cylindrical surfaces and rotating about the axis of the cylinder, in cooperation with a straight counter knife or so-called bottom knife parallel to said axis. In such earlier devices the nap was cut at the points of contact between the helical edges or so-called spirals and the counter knife. The output capacity was dependent on the number of spiral knives on the cylinder, the rotary speed of the cylinder, and the linear speed of the fabric. These parameters controlled the output of the machine and the characteristics of the cropping of the nap of the fabric.
Certain interrelations and limitations have been found in the construction and operation of such machines for cropping the nap of textile fabrics. The number of helical knives which can be provided on a cutting cylinder of given diameter and length is limited, as certain distances must be maintained between the helixes. The peripheral velocity of the cylinder is also limited by a number of factors including the fact that the helical knives act as air blowing fans which produce blowing effects on the nap of the fabric tending to depress this nap and thereby interfering with the proper engagement of the nap by the knives. Difficulties were also encountered since the helical knives were in metal-to-metal contact with the counter knife, except at the points where nap fibers were engaged between the knives. It has been usual to provide the knife edge with a thin lubricant film; however, rapid rotation of the cutting cylinder led to destruction of this film and to consequent rapid wear and tear of the knife edges as well as the development of high temperatures which could even cause burning out of the knife edges.
These difficulties were regrettable, since the economy of cropping or shearing machines depends on the amount of fabric handled by unit of time. If the travelling velocity of the fabrics were increased without regard to the aforementioned problems poor cropping and poor quality of the fabric would result.
Other problems of conventional cropping and shearing machines will be noted hereinafter.